The distortion factor meter or indicator indicates the ratio of the amplitude of a harmonic component after eliminating a fundamental therefrom, to the amplitude of our original signal to be measured. The meter should be corrected so as to adjust the level of the signal to be measured to the reference point on the scale of the indicator when the distortion factor is measured by the indicator. However, this level correction results in the troublesome operation.
There has been proposed a method for treating a signal to be measured, by control means such as an automatic gain control circuit (AGC circuit) after the amplification of the signal to be measured, so as to automatically correct the level of the input signal. This method, however, cannot precisely measure the distortion factor because an additional distortion having no relation to the original signal to be measured is added to the original distortion due to the AGC circuit.
A method has been proposed which is adapted to measure the analogue values of a distortion component and an input signal wave component by means of an analogue multiplier. This method can eliminate the affect from the AGC circuit as well as the troublesome level correction. The analogue multiplier of the prior art has not only a complicated and expensive construction, but also a poor linearity which causes the error of the indication. In addition thereto, it has a narrow dynamic range, and as a result, a poor reliability.